Mr. Khodorkovsky’s supporters had asked Mr. Medvedev to intervene in the case, which has reinforced widespread concerns that the Russian authorities readily manipulate law enforcement and the judiciary for political purposes. But Mr. Medvedev refused, and Mr. Khodorkovsky’s second trial on embezzlement charges — his current sentence ran to 2011 — appeared to end as it began: as a symbol of Mr. Medvedev’s inability to make significant progress on his pledges.

“I expected a guilty verdict, but I didn’t expect such a tough sentence,” said Leonid Y. Gozman, co-chairman of Right Cause, a liberal party with close ties to the Kremlin. “This is shocking. It was obviously a political, not a judicial, decision. A verdict this harsh is going to resonate and be perceived very negatively for Medvedev, and for what he has been trying to accomplish.”

While the Khodorkovsky trial has attracted worldwide attention, the everyday failings in the Russian legal system are pervasive. Nearly two decades after the collapse of Communism, corruption is endemic, government power is often abused and senior politicians are rarely, if ever, held accountable for misdeeds. A series of murders of well-known human rights advocates and journalists have gone unsolved, even as critics of the government are prosecuted.

Yet, with most of the news media cowed by official pressure, and opposition groups largely suppressed, people are skeptical about the possibility of attaining redress, whether through Parliament, the press or the courts.

Mr. Medvedev has conceded that the country is plagued by “legal nihilism,” but Mr. Putin has seemed less bothered. In fact, the new sentence for Mr. Khodorkovsky — which will keep him in prison until 2017 — was considered an unambiguous signal that Mr. Putin, now prime minister, remains in control of the country in advance of a presidential election in 2012, which he might enter.

Mr. Putin, who was president from 2000 to 2008, has repeatedly declared that Mr. Khodorkovsky, 47, is nothing more than a violent criminal who deserves his confinement in a Siberian prison. Mr. Khodorkovsky’s lawyers maintain that the charges against him were fabricated and that he is being persecuted for challenging Mr. Putin by daring to finance opposition political parties.

On Thursday, Mr. Khodorkovsky smiled wanly from behind a glass enclosure as the judge, Viktor Danilkin, handed down the sentence, a scene shown in a video feed from the proceedings in Moscow. His mother, Marina, who was in the courtroom, shouted at the judge, “Damn you and your descendants!”

The judge, who earlier this week found Mr. Khodorkovsky guilty of embezzling billions of dollars worth of oil from his own conglomerate, said Mr. Khodorkovsky could be reformed only through “isolation from society.” Mr. Khodorkovsky, once the country’s richest man, is already serving an eight-year sentence on an earlier conviction that ends in 2011, right before the next presidential election.

The judge made it clear that he agreed with prosecutors that Mr. Khodorkovsky deserved the maximum penalty of 14 years. The new term will be dated from his arrest in 2003, which means that he would be released in 2017. As a result, the new sentence amounts to six years.

Mr. Khodorkovsky’s co-defendant and business partner, Platon L. Lebedev, received a similar sentence. “Platon Lebedev and I have shown by example that you cannot count on the courts to protect you from government officials in Russia,” Mr. Khodorkovsky said in a statement released by his lawyers.

Neither Mr. Putin nor Mr. Medvedev offered any immediate reaction to the sentencing, and it appeared that the Kremlin was determined not to spotlight it. The main news program on the state television channel on Thursday night featured a ceremony at the Kremlin where Mr. Medvedev handed out state honors. Only well into the program was Mr. Khodorkovsky’s sentencing mentioned.

Foreign governments, though, quickly issued statements critical of the sentence, saying it damaged confidence in Russia’s judicial system and sent a warning to foreign investors.

“The impression remains that political motives played a role in this process,” said the German chancellor, Angela Merkel, who is one of the Kremlin’s strongest allies in the West. “This contradicts Russia’s repeatedly expressed intention to follow the path toward a full rule of law.”

In Washington, the State Department repeated a White House comment from this week, condemning what it referred to as “an abusive use of the legal system for improper ends.”

The Russian Foreign Ministry has rejected such criticism, saying that Mr. Khodorkovsky was accused of crimes that would be severely punished in any country. The ministry bluntly added, “We hope that everyone will mind his own business — at home and internationally.”

On Thursday night, most ruling party politicians followed the Kremlin’s lead and avoided commenting on Mr. Khodorkovsky. But a member of Parliament from Mr. Putin’s United Russia party, Yevgeny A. Fedorov, told Echo of Moscow radio that the case actually demonstrated the integrity of Russia’s court system.

“This shows that any lawbreaker, whether the richest man in the country or a member of Parliament or a bureaucrat, will be prosecuted,” he said.

Political experts here said the fate of Mr. Khodorkovsky had been intertwined with jockeying before the 2012 election. They said Mr. Putin did not want Mr. Khodorkovsky released before then, and also wanted to ensure that other wealthy businessmen understood that they should not interfere in politics.

Mr. Putin, who served two terms as president before being barred by the Constitution from a third consecutive one, is considering a candidacy in 2012. He and his protégé, Mr. Medvedev, who rule as a tandem, have said they will decide between themselves who will run, and will not compete against each other.

Mr. Khodorkovsky earned his fortune in the oil industry in the 1990s after the collapse of the Soviet Union, when a generation of entrepreneurs scooped up government assets at a fraction of their worth. Even his supporters acknowledge that he may have committed some white-collar crimes back then, but they insist that he was no different from scores of other so-called oligarchs during a chaotic era. (Mr. Putin has said Mr. Khodorkovsky’s associates killed many people to advance his interests.)

Mr. Khodorkovsky’s supporters said that he was prosecuted because he became politically active, in the early part of this decade, and rebuffed warnings from Mr. Putin and his aides to not disobey the Kremlin.

By the time he was arrested in 2003, Mr. Khodorkovsky had been seeking to refashion his image, saying that his business dealings would be as transparent as any in the West and calling for a more progressive government. He was convicted of tax fraud in 2005. In the current case, he was accused of stealing $27 billion in oil from his conglomerate through accounting schemes that prosecutors said were missed by auditors. His lawyers called the new charges absurd.

A version of this article appears in print on December 31, 2010, on page A1 of the New York edition with the headline: RUSSIA EXTENDS PRISON SENTENCE Of TYCOON 6 YEARS. Order Reprints|Today's Paper|Subscribe